I ask them whether they ever tried learning another language. Usually the answer is yes and usually the language was never learned well. Then I tell them about the studies showing that learning Esperanto as a first foreign language gave enough ‘language learning’ meta-knowledge that it allowed to learn the next language faster and more thoroughly. This usually gets their attention!

I don’t think I have convinced anyone to learn Esperanto yet, but my arguments are getting better every time. Maybe I should make a badge Learn two languages for the price of one. Ask me how. That might get some attention.